1. Field of Invention
This invention relates to monitoring biological activity and growth on objects in contact with aqueous solutions. More specifically, the invention relates to measuring the activity and growth of bacterial films on the surfaces of pipes and equipment containing or transporting water.
2. Description of the Related Art
Many surfaces immersed in water or a substance containing water will soon be coated with a biological growth. The process starts with adsorption of organic material and continues through colonization of bacteria, development of bacterial or algal slime films to the growth of various plants and animals. The development rate can be especially fast in seawater where the first film covering is completed in about two hours and a complete bacterial slime film is formed between 24 and 48 hours after first immersion. (Metals Handbook, Ninth Edition, Vol. 13, pages 900-901, ASM International 1987).
Uncontrolled biological growth increases the resistance of flow through conduits, pipes, heat exchangers, and condensers; and will ultimately block the passageway completely. Restriction of flow generally reduces the capacity of the process using the water. Complete blockage of flow requires costly down time to dean the structure. In addition, biological growth accelerates corrosion of the structure through the mechanisms of microbiolocally influenced corrosion (MIC).
Methods have been used to slow MIC and biological growth in water systems. One approach has been to coat the exposed surfaces of the structure with a biocide bearing or anti-fouling material. Paints containing cuprous oxide or organo-tin are the most common type of anti-fouling coating. A significant drawback to the coating approach is that periodically the coatings must be renewed after the toxic elements leach into the water stream or are otherwise dissolved. The toxicity of the coatings has prompted many of them to be banned. Another problem with this approach is that coatings must be meticulously applied under controlled environment or the coating will peel off in the harsh environment of turbulent flow, thus leaving the structure unprotected against further growth and electrochemical corrosion. Replacement of coatings involves substantial labor for cleaning and preparing the surface and applying the coating. Unless there are back-up systems, coating of critical piping and components result in expensive plant downtime. In critical systems such as emergency cooling water systems in nuclear power plants, coatings are disfavored because of the risk that improperly applied coatings may come off in sheets thus plugging the cooling water source during an emergency.
A commonly used approach to controlling biological growth is to inject a biocide such as chlorine or ozone into the water to kill the bacteria. Since chlorine has a detrimental effect on the environment, strict federal regulations limit the amount of chlorine which can be injected. Thus, it is vitally important to inject chlorine only when necessary. In present piping systems, there is no practical way to inspect or monitor biological activity and growth. Consequently, chlorine is injected sufficiently frequently and in sufficient amounts to insure a clean system. However, the amount of chlorine required to control biological growth varies considerably based on the layout of the equipment, the location of injection points and seasonal changes in bacteria growth rates. Therefore, the biocide injection approach usually results in overdosing which has an adverse impact on the environment and system operating costs. In some cases, expensive dechlorination systems have been added at cooling water discharges to allow high levels of chlorine dosage at critical components and still meet chlorine discharge regulations. To eliminate this problem, an effective method of monitoring biofilm activity and growth is needed so that chlorine injection can be limited to that amount and that periodicity which is effective in preventing the build up of biological growth.
The existing technology to monitor biofilm activity in plants requires exposure of rectangular or small button coupons to the environment of concern, removal of the coupons for inspection, and bacteria cultivation or enzyme analyses. This technology is time consuming and does not provide information fast enough, for instance, to adjust the chlorinators before the biofilm gets established and becomes resistant to chlorine. Other existing methods monitor biofouling in the tubes by heat transfer loss, increase of the water pressure loss or decrease of flow through the tubes. However, these systems indicate biofouling only after a significant buildup. As M. Bibb noted in her article "Bacterial Corrosion," Corrosion and Coatings South Africa, October, 1984, "Once the organism is established beneath the tuberculous crust, huge concentrations of chlorine are required to penetrate this crust." Thus, remedial action under these conditions involves substantial release of chlorine to the environment as opposed to that required when the biofilm is a thin layer.
An existing electrochemical corrosion rate meter is described in U.S. Pat. No. 3,661,750. Electrochemical corrosion probes are shown in U.S. Pat. Nos. 3,633,099 and 3,639,876. A liquid water sensor useful for indicating a corrosive condition in natural gas pipelines is described in U.S. Pat. No. 4,506,540. All the existing electrochemical methods measure data related to corrosion. Electrochemical methods, such as linear polarization, corrosion potential monitoring, polarization testing to determine pitting and repassivation potentials, and AC impedance spectroscopy, have been used with success for monitoring of general corrosion and an indication of pitting corrosion in the plants. These electrochemical methods are not sensitive enough to determine biofilm activity; the instrumentation is complex and expensive, and the data analysis is complicated. The existing monitoring technologies are not practical nor sensitive enough to provide online monitoring of active biofilm formation and MIC in a plant.
Enzyme electrodes, used for instance to measure concentration of glucose in blood or urine, use electrochemical probes with porous graphite/platinum/enzyme electrodes polarized by 325 to 650 mV. The enzyme electrodes operate as amperometric sensors; that is a higher concentration of glucose is indicated by higher DC current flow. An enzyme probe is described in U.S. Pat. No. 4,970,145. The enzyme probe is not sensitive to biofilm activity in a plant and does not work in flowing water applications.